The present invention relates, in general, to an electronic package for mounting an electrical element, such as a transistor, amplifier, or semiconductor chip, including a structure having an opening therein for substantially preventing flow of a bonding adhesive onto an area adapted for mounting the electrical element.
Many modern high power radio frequency or microwave applications require significant current carrying capability and/or significant thermal dissipation. To meet these needs, a thermally conductive member, such as thick metal backer (TMB) is often connected to an external ground plane located on the surface of a substrate, such as a printed circuit board (PCB), to improve the electrical performance of the ground plane and also to provide a heat sink for thermal dissipation. The TMB is essentially flat, but may contain a variety of holes and partial depth cavities for mounting various types of electrical elements, such as transistors, amplifiers, or semiconductor chips. These may or may not be hidden once the printed circuit card is attached. Methods for connecting such a TMB to the external ground plane of the PCB have included sweat soldering, non-conductive adhesive bonding followed by plating through holes for electrical interconnectivity, gold coating the ground plane and applying a silicone adhesive which includes a mixture of silver particles, or by mechanical interconnection such as screws, rivets or soldered pins. Each of these methods have proven less than ideal due primarily to relatively high costs.
Conductive bonding of the PCB to the TMB has cost and performance advantages, but achieving long term reliability of the conductive bond has been made more difficult because the TMB is typically made of aluminum. Aluminum does provide an excellent combination of weight, stiffness, machinability, electrical and thermal conductivity, and cost. However, the oxide that forms on the surface of aluminum results in relatively poor electrical conductivity when connected with electrically conductive adhesives, and poor stability in extended testing at elevated temperature and humidity has been observed. To overcome this problem, a more recent method, described in U.S. Pat. No. 5,920,037 xe2x80x9cConductive Bonding Design for Metal Backed Circuitsxe2x80x9d, commonly assigned and incorporated by reference herein, meeting the current carrying and/or thermal dissipation needs required for microwave applications involves treating the surface of the TMB and then bonding the treated surface of the TMB to the back side of the PCB with a conductive adhesive. This method includes the steps of preparing a bonding surface of the TMB by micro-roughening the bonding surface. A conductive adhesive is screened onto the TMB at a thickness of about 0.003 inches (3 mils). The back side of the PCB is placed onto the conductive adhesive on the TMB and a roller is rolled across the PCB to insure intimate contact and wetting of the PCB with the adhesive. This electronic package assembly formed by joining the TMB with the PCB is then placed under pressure in a clamping fixture with pressure exerted at about 1 pound per square inch (psi). The electronic package contained within the fixture is placed in a conveyorized IR (infrared) oven for curing. After curing, the electronic package is removed from the clamping fixture.
While the micro-roughening method improves the adhesive to TMB interface strength, the low pressure, typically about 1 psi., used during the conductive adhesive joining and curing processes may not be sufficient to result in a void free bond line. This is principally due to pockets of air that are trapped in the bondline after the PCB is positioned on the TMB. The lamination pressure of 1 psi. is insufficient to force air from the bondline. For instance, the backside of the PCB may be comprised of bare dielectric material in some places, but have circuit features that are as much as 2 mils above the dielectric. Moreover, a 2 mil protective coating of solder mask such as PSR-4000 (PSR-4000 is a registered trademark of Taiyo America Inc., 2675 Antler Drive, Carson City, Nev.) or solder mask as described in U.S. Pat. No. 5,026,624 xe2x80x9cComposition for Foil Imagingxe2x80x9d, commonly assigned and incorporated by reference herein, may also be present in selected areas on the backside of the PCB, including areas where there are circuit features. The resulting topography in this example may therefore be as much as 4 mils above the backside surface of the PCB. Thus, in the particular example when the surface topography of the PCB is comparable to the bondline thickness (3 mils) of the adhesive, voids in the adhesive bondline are possible when using known methods.
Prior attempts to overcome this problem of voiding due to surface topography have not been satisfactory. For example, the use of higher pressure during the process of laminating the PCB results in excessive squeeze out (bleed) and flow of the adhesive. If the edge bleed is significant, this adhesive flow can damage the assembly by contaminating surface features adapted for subsequent use as amplifier, transistor or semiconductor chip attachment pads. As another alternative, the use of a vacuum bagging technique to provide good conformance of the PCB to the TMB has been attempted, but also results in excessive bleed of the adhesive. In another approach, a thicker layer of adhesive is used, but even lower pressure is then required to prevent excess bleed. Moreover, this approach is undesirable because of the higher cost of the adhesive, and the increase in electrical resistance associated with a thicker bondline.
Previously, it was found that partially curing a conductive adhesive after the adhesive has been screened onto a TMB, but before attaching the PCB, can reduce the formation of trapped air and voids. This partial curing of the adhesive along with putting the PCB/TMB assembly under high pressure allowed a thinner bond line to be screened onto the TMB than the topography of the PCB. Even though partial curing the adhesive reduced the amount of bleed, there is still enough bleed to cause electrical connection problems under some circumstances, thus resulting in yield problems in the manufacturing process to assemble electrical elements to the TMB.
The present invention is directed at overcoming the problems set forth above. It is desirable to have an electronic package and method to make the electronic package that will substantially prevent adhesive that bleeds from under the PCB from contacting surface features adapted for subsequent use as amplifier, transistor or semiconductor chip attachment pads. Electronic packages produced by this method will have increased yield, lower cost, and improved operational life, since electrical elements can be bonded to surface features without the presence of conductive adhesive.
Accordingly, it is an object of this invention to enhance the art of packaging technology.
It is another object of this invention to provide a novel method for producing an electronic package that substantially prevents adhesive that may bleed from under a substrate bonded to a thermally conductive member from contacting a substantially planar first portion on a surface of the thermally conductive member of the electronic package.
It is yet another object of this invention to provide an electronic package that will be manufactured with increased yields and can be produced at relatively lower costs than many current products.
Still another object of this invention is to provide an electronic package adapted for having an electrical element or the like positioned on a substantially planar first portion of a surface of a thermally conductive member. The package substantially prevents adhesive that may bleed from under a substrate bonded to the thermally conductive member from contacting the substantially planar first portion on the surface of the thermally conductive member, resulting in a package having much improved operational field life.
According to one aspect of this invention, there is provided an electronic package, comprising a thermally conductive member having a first surface and an opening located therein, the opening including at least one side wall and a bottom wall. The first surface of the thermally conductive member includes a substantially planar first portion adapted for having an electrical element or the like positioned thereon and a second portion. The electronic package further includes a quantity of adhesive positioned on the second portion of the first surface of the thermally conductive member relative to the opening, and a substrate positioned on the quantity of adhesive. The adhesive securedly holds the substrate in position on the thermally conductive member, and the opening substantially prevents the adhesive from contacting the substantially planar first portion of the first surface prior to positioning of the electrical element on the substantially planar first portion.
According to another aspect of the invention there is provided a method of making an electronic package comprising the steps of providing a thermally conductive member having a first surface including a substantially planar first portion adapted for having an electrical element or the like positioned thereon and a second portion, and providing an opening within the first surface, the opening having at least one side wall and a bottom wall. The method also includes applying a quantity of adhesive on the second portion of the first surface of the thermally conductive member relative to the opening. The method further includes positioning a substrate on the quantity of adhesive, the adhesive securedly holding the substrate in position on the thermally conductive member, the opening substantially preventing the adhesive from contacting the substantially planar first portion of the first surface prior to positioning of the electrical element on the substantially planar first portion.
The above objects, advantages, and features of the present invention will become more readily apparent from the following detailed description of the presently preferred embodiments as illustrated in the accompanying drawings.